Fix wait in networkIdle when event already fired

We need to register that the `networkIdle` event has fired, just as
we're doing for the `load` and `DOMContentLoaded`. We sometimes see
that the `networkIdle` event is fired before the frame has a chance
to register a listener/handler.

common/frame_session.go

@@ -721,6 +721,8 @@ func (fs *FrameSession) onPageLifecycle(event *cdppage.EventLifecycleEvent) {
 		fs.manager.frameLifecycleEvent(event.FrameID, LifecycleEventLoad)
 	case "DOMContentLoaded":
 		fs.manager.frameLifecycleEvent(event.FrameID, LifecycleEventDOMContentLoad)
+	case "networkIdle":
+		fs.manager.frameLifecycleEvent(event.FrameID, LifecycleEventNetworkIdle)
 	}
 
 	eventToMetric := map[string]*k6metrics.Metric{